The Weston Clean PAL system is described in GB Nos. 1534268 to 1534270 and in BBC Research Department Report No: 1980/1. The contents of these four documents are incorporated herein by reference. W-PAL obtains separation of luminance and chrominance by phase segregation, whereby the luminance is sampled at a frequency of twice the colour subcarrier frequency thereby causing it to appear as double sideband modulation of the colour subcarrier. At the same time the two chrominance components are formed into a single signal consisting of the quantities (U+V)/.sqroot.2 and (U-V)/.sqroot.2 on alternate lines which modulates the subcarrier directly. If PAL compatibility were not required the two modulation processes could be in quadrature so enabling the subcarrier to carry both luminance and chrominance without interaction in spite of their occupying the same spectral space. FIG. 1 shows the two-dimensional spectrum of the alternate-line chrominance and the sampled luminance, from which it can be seen that there is a considerable overlap of the luminance and chrominance spectra, amounting to the region 3.36-5.5 MHz in system I PAL. This region is the region in which aliasing will occur at 2 Fsc sampling (8.86 MHz-5.5 MHz=3.36 MHz) and varies with different PAL standards.
Compatibility with normal PAL, however, requires that the signals are further processed before combination. In particular, the combined chrominance signal must be phase shifted so that spectral components at 3/4 line offsets are in phase quadrature with components at 1/4 line offsets, restoring the phase quadrature between U and V. This must be done in such a way as to preserve the potential phase separation between chrominance and luminance. At the receiver, the UV phase separation is removed and the luminance and chrominance are separated by synchronous demodulation. These complex functions are provided by so-called assembler and splitter circuits which lie at the core of the Weston Clean PAL system. These circuits are described in GB Nos. 1534268 and 1534270.